1. Field of the Invention
The present invention relates to the partial tempering of rolled wire products made of steel.
2. The Prior Art
In the production of bars, quenching directly from the rolling heat, and thus partially tempering the steel, is known. With bars of finite lengths, the bars, after being cut, are moved axially and in so doing are treated with a cooling medium. This method has gained wide application and has proved itself many times.
In the partial tempering of rolled wire, this method cannot be used without great difficulty, because at the typical rolling speed and with a necessary cooling period of from 0.5 to 1 s, the cooling line can be up to 120 m long. Such a long cooling line is vulnerable to malfunction, and both an initial and a final length of the rolled product remain uncooled, so there is considerable waste.
The term "partial tempering" is understood to mean a process that affects only the peripheral region of the tempering material, in which the peripheral zone is converted completely to martensite and after the quenching process is ended is annealed again by the residual heat present in the center of the cooling material. The important factor is the peripheral zone is converted completely and homogeneously to annealed martensite.
It is known for wires to be cooled down first from the heat of the rolling mill, and then delivered to a winding layer. Examples of this are described in British patent application No. 1,128,35, German examined patent application No. 1,262,323 and German unexamined patent application No. 1,452,343. Such methods were offered in order to improve the cold drawing capability. The hot-rolled wire here is subjected to an intermediate-stage tempering, and martensite formation during the cooling process is very consciously avoided, because it considerably impairs the drawing capability.
In contrast to that, German unexamined patent application No. 2,829,428 discloses an apparatus for producing wires usable for concrete reinforcing bars. Here, following the final rolling pass, cooling to approximately 750.degree. C. is supposed to take place, so that after the following winding layer the cooling can be completed to 600.degree. to 500.degree.. As indicated in this published reference, a complete conversion of the peripheral region to a martensitic structure is not sought; this is dictated by the first cooling stage, in which the precipitation of perlite already takes place, and only the remaining amount of austenite can be converted into martensite.